In order to overcome the limitations of conventional silicon-based inorganic devices, advanced materials such as nanowires, carbon nanotubes and conductive polymers have been studied on various fields.
Particularly, graphene that was recently found is a planar sheet of carbons arranged in a hexagonal pattern and shows specific physical properties such as quantum hall effects. Also, a graphene layer has a very high electrical conductivity at a transmittance of 97.5%, and thus is expected to substitute for a transparent electrode of indium tin oxide. (ITO), the price of which is increasing rapidly. In addition, due to its flexible property, graphene can be used as transparent electrode materials and semiconductor materials in devices that are required in the industry.
The key to use graphene in industrial applications is to ensure technology for producing large amounts of graphene. A method of detaching graphene using a tape has the advantage of obtaining good quality graphene, graphene that is obtained by this method is only several micrometers in size, and thus cannot be used in industrial applications. In addition, a method of fabricating graphene sheets by dispersing graphite oxide (obtained by treating graphite with strong acid) in a solvent such as water causes problems, such as formation of various functional groups in graphene, which make it impossible to maintain the excellent electrical properties of graphene.
It is expected that a chemical vapor deposition (CVD) method of synthesizing graphene in a vapor phase using a catalyst such as copper (Cu) or nickel (Ni) can synthesize large amounts of good-quality graphene, and thus studies on the synthesis of good-quality graphene using this method are being actively conducted. However, graphene synthesized using the chemical vapor deposition method has a polycrystalline structure and shows inferior properties compared to graphene having a single-crystalline structure. This is because the domain boundaries of graphene greatly influence the electrical and mechanical properties of graphene.
As described above, it is very important to observe the domains and domain boundaries of graphene in order to synthesize graphene having ideal properties. However, conventional methods, including Raman 2D mapping, low-energy electron diffraction and transmittance electron microscopy, use expensive systems, require a large amount of time to observe graphene domains and are only effective for domains that are less than a few micrometers in size. Thus, there is a need for a method of observing the domains and domain boundaries of graphene in a general and easy way in order to control the properties of graphene.
Korean Patent Laid-Open Publication No. 2010-0112726 discloses a method capable of detecting the edge shape of graphene nanoribbons using an atom or a molecule, which shows higher adsorption energy at the zigzag edge of graphene than that at other portions. However, this method has problems in that the results of analysis can differ depending on whether the atom or the molecule is adsorbed, only the edge shape of graphene nanoribbons can be measured, and the measured edge shape of nanoribbons cannot be visualized.
Accordingly, the present inventors have made extensive efforts to observe the domains and domain boundaries of graphene in large area in a general and easy way and, as a result, have found that the domains and domain boundaries of graphene can be easily optically visualized by forming on a substrate a graphene layer to be measured, forming a liquid crystal layer on the formed graphene layer, and then measuring the optical properties of the formed liquid crystal layer, thereby completing the present invention.